Dedicated to Professor Josef Michl on the occasion of his 70 th birthday
In general, silylenes (R 2 Si, in which R = H, halogen, alkyl, or aryl) are divalent neutral silicon species with the lone pair of electrons as the HOMO and an empty p orbital as the LUMO in the singlet ground ( 1 A) state. Therefore, silylenes can in principle behave as Lewis acids as well as Lewis bases, depending on the substituents. Silylenes are known to have an ambiphilic [1] nature (A and B in Scheme 1). The availability of simultaneously electrophilic and nucleophilic center in the same molecule such as a silylene (A), has been intriguing both to theoretical [1] and experimental [2] scientists during the last few decades. A fair number of examples of insertion or addition reactions of stable silylenes have been reported. [3] The reactions in which silylenes function as Lewis bases are limited. [4,5] The base-catalyzed generation of silylenes as reaction intermediates and their trapping products are known. [6] Many transient silylene complexes with Lewis bases were postulat-ed in low-temperature matrices, [7] but the first stable silylene Lewis base complex was isolated by Okazaki et al. in 1997 by the reaction of disilene with isocyanide. [8] A variety of transition-metal-silylene complexes [9] are reported in the literature in which they exhibit metal-silicon multiple bond character. Very recently, we isolated and reported on stable N-heterocyclic carbene (NHC)-base-stabilized dichlorosilylenes [10a] L!SiCl 2 (1; in which L = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene in 1 or 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene) under very mild reaction conditions. Subsequently, NHC-stabilized dibromosilylene was also reported. [10b] The NHC in 1 behaves as a s-donor ligand and the stereochemically active lone pair resides at the silicon atom. A N-heterocyclicsilylene-borane adduct [5] [(CHtBuN) 2 Si]!BA C H T U N G T R E N N U N G (C 6 F 5 ) 3 , with a half life of one month to convert to silylborane is known, but it could not be characterized by X-ray diffraction. Furthermore, the reaction of the silylene-isocyanide complex with borane resulted in the formation of a silylborane-isocyanide complex [11] instead of a silylene-borane adduct. Herein we report on the use of NHC-stabilized dichlorosilylene L!SiCl 2 (1) as a base in the reaction to give the silylene-borane adduct L!SiCl 2 ! BA C H T U N G T R E N N U N G (C 6 F 5 ) 3 (2). Adduct 2 is a novel example showing both acidic and basic properties of dichlorosilylene in a single molecule, as a s-acceptor as well as a s-donor.
The reaction of 1 and BA C H T U N G T R E N N U N G (C 6 F 5 ) 3 in toluene (Scheme 2) and subsequent concentration of the resulting solution afforded colorless crystals of 2 on storage at À35 8C for two days.
Compound 2 is soluble in common organic solvents and is stable under inert gas atmosphere; it has been characterized by elemental analyses, 1 H, 11 B, 13 C, 19 F, 29 Si NMR spectroscopic studies, and EI mass spectrometry. The molecular structure of 2 was established by single-crystal X-ray diffraction and DFT calculations with subsequent topological analysis.
The 1 H NMR spectrum of 2 exhibits resonances for the NHC ligand bound to the Si atom (d = 0.80 (CHMe 2 ), 1.15 (CHMe 2 ), 2.62 (CHMe 2 ), 6.29 (NCH), 6.89 (m-C 6 H 3 ), 7.02-7.07 ppm (p-C 6 H 3 )) with upfield shifted resonances relative to 1 (d = 1.01 (CHMe 2 ), 1.43 (CHMe 2 ), 2.79 (CHMe 2 ), 6.36 [a] Dr.